Chromatic aberration in imaging systems typified by camera lenses and the like is corrected by combining high dispersion lenses and low dispersion lenses. In recent years, to the extent that aberration can be corrected, the tendency has been to employ optical glass of higher refractive index in both high and low dispersion lenses to achieve greater functionality and greater compactness.
It is possible to achieve greater compactness and functionality in an optical system by employing an optically functional surface on an aspherical lens in addition to the optical characteristics of the glass constituting the lens. Precision press molding is suited to the production of such lenses in large quantities.
To increase the productivity of precision press molding, it is first necessary for the glass material or preform for precision pressing to have a certain moldability to ensure production properties. That is, a certain level of glass viscosity at a temperature at which crystals do not precipitate or a certain resistance to devitrification at a temperature where the glass has a viscosity permitting molding is required.
In addition, it is necessary to increase the productivity of precision press molding. An expensive mold material having resistance to heat and high rigidity, such as SiC or a carbide material, and a pressing mold the molding surface of which has been precision processed into a shape that is the reverse of the shape of the optically functional surface of an optical element, are employed in precision press molding. Thus, the service life of the pressing mold greatly affects production costs and large-quantity productivity. Thus, effort is expended on employing a glass with a low glass transition temperature and a low sag temperature to lower the temperature during press molding and avoid thermal degradation of the pressing mold.
Patent Documents 1 and 2 propose low-dispersion glasses for use in precision press molding.    [Patent Document 1] JP-A-2006-016293    [Patent Document 2] JP-A-2006-016295
The descriptions in above-mentioned Patent Documents 1 and 2, and of US2005-0272589A1 and US2008-0220961, which are members of the family of Patent Documents 1 and 2, are hereby incorporated by reference in their entirety.